REPRODUCTION OF SPONGES. 



Fig. 12. 



from end to end, subsequently quitting it ; or else, assuming its globular 

 form, it will embrace some part of the spiculum and remain stationarily 

 attached to it. The changes in shape and position of the sponge-cell 

 are for the most part effected so imperceptibly that they may be likened 

 to those which take place in a cloud. Its granules, however, are more 

 active ; but there appears to be no motion in any part of the cell (ex- 

 cepting among the molecules within the hyaline vesicle) which in any 

 way approaches to that characteristic of the presence of cilia. 



(57.) The intercellular substance that forms the bond of union be- 

 tween the sponge-cells is of a mucilaginous appearance. When observed 

 in the delicate pellicle which, with its imbedded cells, it forms over 

 the surface and throughout the canals of the sponge, it is transparent ; 

 but when a portion of this pellicle is cut off from its attachments, it 

 collapses and becomes semi-opake. In this state the detached portion 

 immediately evinces a tendency to assume a spheroidal form; but whether 

 the intercellular substance participates in this act or remains passive 

 while the contraction is wholly performed by the habit of the cells im- 

 bedded in it to approximate themselves, is not evident. 



(58.) The freshwater sponges are 

 reproduced from seed-like bodies found 

 in the substance of the oldest or first- 

 formed portions of the sponge, never in 

 its periphery. They are round or ovoid 

 according to the species, and each pre- 

 sents a single infundibular depression 

 on its surface which communicates 

 with the interior. At the earliest pe- 

 riod of development at which it is re- 

 cognizable, it is composed of a number 

 of cells united together by an intercel- 

 lular substance similar to that de- 

 scribed above. In this state, appa- 

 rently without any capsule, and about 

 half the size of the fully developed 

 seed-like body, it seems to lie free in a 

 cavity formed by a condensation of the common structure of the sponge 

 immediately surrounding it. The cells of which it is now composed ap- 

 pear to differ from those of the fully-developed sponge-cell only in being 

 smaller, in the colourless state of their contained granules, and in the 

 absence of hyaline vesicles. The seed-like body gradually passes from 

 the state just mentioned into a more circumscribed form, then becomes 

 surrounded by a soft, white, compressible capsule, which finally thickens, 

 turns yellow, and developes upon its exterior a firm crust of silicious 

 spicula, presenting in some species a hexagonally tessellated appearance 

 (fig. 12, c). The spicula are arranged perpendicularly to the surface of 



Magnified section of a seed-like body of 

 Spongilla Meyeni, showing, f, spicular 

 crust; g, coriaceous capsule; k, internal 

 cells ; i, infundibular opening, c, portion 

 of coriaceous membrane, magnified, to 

 show the hexagonal divisions with trans- 

 parent centres ; d, small spiculum, mag- 

 nified; e, one of its toothed disks with 

 central aperture, magnified. (After Mr. 

 H. J. Carter.) 



